Through bioinformatics analyses of a comprehensive database of gene expression (Body Index of Gene Expression (BIGE)), we sought to identify novel/uncharacterized genes associated with the immune system. We identified a poorly characterized secreted protein expressed by activated macrophages encoded by a gene currently identified as ?meteorin-like? (metrnl). This gene is strongly expressed in barrier tissues (skin, mucosa) and by activated macrophages. Metrnl encodes a small secreted protein of 311 aminoacids with a 45 aminoacid signal peptide, predicting a mature protein of 266 aminoacids, with an estimated molecular weight of ~29KDa. It is expressed in human peripheral blood monocytes or mouse peritoneal macrophages following activation and its expression is modulated (positively or negatively) by various cytokines. We have produced a Metrnl/IL-39-/- mouse which is viable and breeds well, but exhibits various immune system defects. Taken together, we conclude that Metrnl represents a novel cytokine for which we recommend the name interleukin 39 (IL-39). Metrnl/IL-39 is overexpressed in psoriasis and rheumatoid arthritis, suggesting a role in human inflammatory/autoimmune diseases. Metrnl/IL-39 expression is also induced in the peritoneum of mice injected with thioglycollate, consistent with a role in inflammation. The immune system abnormalities we have detected in the Metrnl/Il-39-/- mouse, include lower serum IgG levels, which are due to very low levels of IgG2b and IgG3. Furthermore, its splenocytes exhibit altered cytokine production, including inability to produce some chemokines (CCL3) as well as significantly lower levels of IFN?. These observations strongly support our conclusion that Metrnl/IL-39 represents a novel cytokine involved in immunoregulation. Metrnl/IL-39 has been reported to induce the expression of the nuclear receptor PPAR? in adipocytes, but its effects on cells of the immune system are unknown. In Specific Aim 1, we will identify cells of the immune system that express the Metrnl/IL-39 receptor based on the ability of Metrnl/IL-39 to induce the expression of PPAR?, or to bind biotinylated Metrnl/IL-39 (detected by FACS). We will also explore whether the B cell defects observed in the Metrnl/IL-39-/- mouse (very low IgG2b and IgG3) are due to defective class switching in B cells, and will explore a potential role for Metrnl/IL-39 in the polarization of nave T cells to the Th1, Th2, Th17 and iTreg stages. We have also observed that the Metrnl/IL-39-/- mouse is highly susceptible to Toxoplasma gondii. In Specific Aim 2, we will explore several immune mechanisms that may account for this increased susceptibility to T. gondii. We will study the ability of Toxoplasma to grow in monocytes from Metrnl/IL-39 or wild type (WT) mice, and will also monitor Th1, Th2 or Th17 cytokine levels in Metrnl/IL-39-/- or WT mice during infection. We will finally determine whether administration of Metrnl/IL-39 can reverse the susceptibility observed in the Metrnl/IL-39-/- mouse to T. gondii. Through these studies we will validate the hypothesis that Metrnl/IL-39 represents a novel cytokine associated with the immune system and inflammation and will open a new field of research.